Comes down to two individuals in my mind, though a lot can change in 4 years. Paul Ryan and Rand Paul. Paul Ryan's star is clearly rising and many conservatives consider him a budgetary hawk. He is also a congressman from the battleground state of Wisconsin and the republicans desperately need to garner votes. As Vox has said, the republicans may have hit a demographic tipping point, and unless the drop in the number of childless and single Americans, voters who overwhelmingly support progressive policies, is great enough then the GOP as we know it is caput.
As for Rand Paul, his fiscal credentials are much heftier than Paul Ryan just compare their budget plans. Rand Paul offered to cut federal spending by $ 500 billion, or a third of the deficit at the time, while Paul Ryan budgeted for increases, along with an increase in government tax receipts. He has a lot more credibility with the libertarian minded Americans, though he admittedly tarnished his image by endorsing Romney, and of the two I think he would be a better president. But that is neither here or there. A lot of things will happen that could change my prognosis on the two front runners in four years time. But I know this one thing is undeniably true, things are only going to get worse, and 2016 is probably our last chance to reverse the tide; if it isn't already too late.